For many animals, life is a cycle of scarcity and plenty. Hibernating creatures cower underground in winter, slowing down their metabolism so they can arrive in spring without food. Even lab mice, if starved of food, can enter a state called torpor, a kind of energy-saving sleep mode.
It’s something humans have long dreamed of: if we ever leave this planet and travel into space, we will experience our own period of scarcity. Science fiction writers tend to imagine mysterious technology that keeps humans in stasis, able to survive centuries of silence before emerging into a new life. For now, it is a technology that is out of reach.
But as scientists strive to understand states like torpor and hibernation, tantalizing details about how the brain controls metabolism have emerged. Researchers have reported in the journal Nature Metabolism Thursday that they were able to send mice into a state of torpor by targeting a specific part of the brain with short bursts of ultrasound. It’s unclear exactly why ultrasound has this effect, but the results suggest that studying the neural circuits involved in torpor may reveal ways to manipulate metabolism beyond the lab.
Ultrasonic devices, which generate high-frequency sound waves, are best known for their imaging powers. But they have also been used by neuroscientists to stimulate neurons. Properly tuned, sound waves can travel deep into the brain, said Hong Chen, a professor of biomedical engineering at Washington University in St. Louis and author of the new paper. In 2014, William Tyler, now at the University of Alabama at Birmingham, and his colleagues applied ultrasound to a sensory region of the brain and found that it improved a subject’s sense of touch. A growing number of works are exploring ultrasound as a treatment for conditions like depression and anxiety.
Curious to learn more about a region of the brain that regulates body temperature in rodents, Dr. Chen and his colleagues built tiny ultrasonic mouse caps. The devices trained six bursts, each consisting of 10 seconds of ultrasound, on the selected area of the rodent’s brain. (Researchers who study the brain with ultrasound must adjust their devices carefully to avoid heat, which can damage tissue).
The mice, the researchers noticed, stopped moving. Measurements of their body temperature, heart rate and metabolism showed a pronounced drop. The mice remained in this state for about an hour after the ultrasound bursts, then returned to normal.
Looking more closely at the neurons involved in this response, the researchers identified a protein in their brain membranes, TRPM2, which appears to be sensitive to ultrasound; when the researchers reduced the protein levels in the mice, the mice became resistant to the effects of the ultrasound.
This is an important step in understanding how ultrasound affects neurons, said Davide Folloni, a researcher at the Icahn School of Medicine at Mount Sinai in New York who studies the brain using ultrasound; details have been largely elusive.
But it’s also possible that the heat generated by ultrasound, not just the ultrasound itself, affects TRPM2 in mouse brains, a point that was raised by Masashi Yanagisawa and Takeshi Sakurai from the University of Tsukuba in Japan. , in separate interviews. The two studied neurons in this region of the brain and their link to states of torpor. Both can be at play, Dr. Chen said.
In one of the most tantalizing parts of the study, the researchers tested whether animals that don’t usually experience torpor – rats – behaved differently when the brain region was stimulated with ultrasound. Indeed, they seemed to slow down and their body temperature dropped.
“We have to be careful with data on rats,” warns Dr. Chen. So far they only have temperature information, not metabolic rate and other factors.
Could ultrasound be a means of modifying the metabolism of larger animals without a history of torpor, such as humans? It’s an intriguing idea, said Dr. Sakurai.
“At this point,” he said, “it remains an unanswered question.”